The present invention relates to an internal combustion engine.
Known is a direct injection type internal combustion engine which forms an air-fuel mixture in a limited region of a combustion chamber and ignites the air-fuel mixture by a spark plug when the engine load is relatively low and which fills the combustion chamber with a uniform air-fuel mixture and ignites the uniform air-fuel mixture by a spark plug when the engine load becomes higher. In this direct injection type internal combustion engine, normally, for example as disclosed in Japanese Unexamined Patent Publication (Kokai) No. 5-18245, the spark plug is arranged at the center of the inner wall surface of a cylinder head, a groove extending from below a fuel injector to below the spark plug is formed in a top surface of a piston, fuel is injected toward the groove when the engine load is relatively low, and the injected fuel is guided by the bottom surface of the groove to form an air-fuel mixture in a limited region around the spark plug.
If fuel is injected from the fuel injector, however, right after injection, an overly rich air-fuel mixture is formed at the center of the fuel mist. Therefore, if the air-fuel mixture is ignited by the spark plug right after fuel injection, the overly rich air-fuel mixture is burned and as a result a large amount of soot is produced. Accordingly, in the past, in direct injection type internal combustion engines, the practice had been to advance the fuel injection timing to cause the injected fuel to disperse before ignition and eliminate the presence of an overly rich air-fuel mixture region around when the mixture was ignited and thereby prevent the generation of soot.
When forming an air-fuel mixture in a limited region in a combustion chamber, however, if advancing the fuel injection timing to cause the injected fuel to disperse in this way, a considerably lean air-fuel ratio region is formed over an extensive area around the air-fuel mixture. If a considerably lean air-fuel ratio region is formed over an extensive area in this way, however, the flame of ignition of the spark plug will not be propagated well in that region and therefore a large amount of unburned hydrocarbons will be produced. That is, the amount of fuel not being burned well will increase, so the problem of an increase in the amount of fuel consumption will arise.
In this case, if delaying the fuel injection timing to ignite the air-fuel mixture before the injected fuel disperses, the flame of ignition will quickly be propagated to the air-fuel mixture as a whole and the air-fuel mixture as a whole will be burned. As a result, almost no unburned hydrocarbons will be produced and the amount of fuel consumption can be reduced. At this time, however, an overly rich air-fuel mixture region will be formed, so as explained above a large amount of soot will be produced.
If a large amount of soot were not produced at this time, no unburned hydrocarbons would be produced and ideal combustion with little fuel consumption could be obtained.
On the other hand, in the past, in internal combustion engines, the production of NOx has been suppressed by connecting the engine exhaust passage and the engine intake passage by an exhaust gas recirculation (EGR) passage so as-to cause the exhaust gas, that is, the EGR gas, to recirculate in the engine intake passage through the EGR passage. In this case, the EGR gas has a relatively high specific heat and therefore can absorb a large amount oft heat, so the larger the amount of EGR gas, that is, the higher the EGR rate (amount of EGR gas/(amount of EGR gas+amount of intake air), the lower the combustion temperature in the engine intake passage. When the combustion temperature falls, the amount of NOx. produced falls and therefore the higher the EGR rate, the lower the amount of NOx produced.
In this way, in the past, it was known that the higher the EGR rate, the lower the amount of NOx produced can become. If the EGR rate is increased, however, the amount of soot produced, that is, the smoke, starts to sharply rise when the EGR rate passes a certain limit. In this point, in the past, it was believed that if the EGR rate was increased further, the concentration of oxygen. around the fuel would fall and result in an overly rich mixture and the smoke would increase without limit. Therefore, it was believed that the EGR rate at which smoke starts to rise sharply was the maximum allowable limit of the EGR rate.
Therefore, in the past, the EGR rate was set within a range not exceeding this maximum allowable limit. The maximum allowable limit of the EGR rate differed considerably according to the type of the engine and the fuel, but was from 30 percent to 50 percent or so. Accordingly, in conventional internal combustion engines, the EGR rate was suppressed to 30 percent to 50 percent or so at a maximum.
Since it was believed in the past that there was a maximum allowable limit to the EGR rate, in the past the EGR rate had been set within a range not exceeding that maximum allowable limit so that the amount of NOx produced would become as small as possible. Even if the EGR rate is set in this way so that the amount of NOx produced becomes as small as possible, however, there are limits to the reduction of the amount of production of NOx. In practice, therefore, a considerable amount of NOx continues being produced.
In the process of studying the combustion in internal combustion engines, however, the present inventors discovered that if the EGR rate is made larger than the maximum allowable limit, the smoke sharply increases as explained above, but there is a peak to the amount of the smoke produced and once this peak is passed, if the EGR rate is made further larger, the smoke starts to sharply decrease and that if the EGR rate is made at least 70 percent during engine idling or if the EGR gas is force cooled and the EGR rate is made at least 55 percent or so, the smoke will almost completely disappear, that is, almost no soot will be produced. Further, they found that the amount of NOx produced at this time was extremely small. They engaged in further studies later based on this discovery to determine the reasons why soot was not produced and as a result constructed a new system of combustion able to simultaneously reduce the soot and NOx more than ever before. This new system of combustion will be explained in detail later, but briefly it is based on the idea of stopping the growth of hydrocarbons into soot at an intermediate stage before the hydrocarbons grow.
That is, what was found from repeated experiments and research was that the growth of hydrocarbons stops at an intermediate stage before becoming soot when the temperature of the fuel and the gas around the fuel at the time of combustion in the combustion chamber is lower than a certain temperature and the hydrocarbons grow to soot all at once when the temperature of the fuel and the gas around the fuel becomes higher than a certain temperature. In this case, the temperature of the fuel and the gas around the fuel is greatly affected by the heat absorbing action of the gas around the fuel at the time of combustion of the fuel. By adjusting the amount of heat absorbed by the gas around the fuel in accordance with the amount of heat generated at the time of combustion of the fuel, it is possible to control the temperature of the fuel and the gas around the fuel.
Therefore, if the temperature of the fuel and the gas around the fuel at the time of combustion in the combustion chamber is suppressed to no more than a temperature at which the growth of the hydrocarbons stops midway, soot is no longer produced. The temperature of the fuel and the gas around the fuel at the time of combustion in the combustion chamber can be suppressed to no more than a temperature at which the growth of the hydrocarbons stops midway by adjusting the amount of heat absorbed by the gas around the fuel. On the other hand, the hydrocarbons stopped in growth midway before becoming soot, that is, the unburned hydrocarbons, are exhausted, but the amount of exhaust of the unburned hydrocarbons is far less than when advancing the fuel injection timing.
Therefore, when delaying the fuel injection timing, if using this new combustion system, even if an overly rich air-fuel mixture region is formed, almost no soot is produced and further almost no NOx is produced. Therefore, ideal combustion in which almost no soot and NOx are produced and the amount of fuel consumption is small can be obtained.
An object of the present invention is to provide an internal combustion engine capable of obtaining this ideal combustion.
According to the present invention, there is provided an internal combustion engine provided with a spark plug for igniting fuel injected into a combustion chamber and in which an amount of production of soot peaks when an amount of inert gas in the combustion chamber is increased if delaying a fuel injection timing in a compression stroke, wherein the amount of inert gas in the combustion chamber is made greater than the amount of insert gas at which the amount of production of soot peaks and thereby the temperature of the fuel and the gas around it at the time of combustion in the combustion chamber is suppressed to a temperature lower than the temperature at which soot is produced, whereby the production of soot is suppressed.